by Douglas Robson, USA TODAY Sports

by Douglas Robson, USA TODAY Sports

Once pigeonholed as a clay-court specialist, Nadal adapted his grinding game to grass and won Wimbledon. He then mastered hardcourts, winning the Australian Open and U.S. Open.

His latest transformation: Comeback King.

"This time last year we were worried if he'd ever play again," said Tennis Channel's Justin Gimelstob, referring to Nadal's seven-month layoff following Wimbledon a year ago. "Now he's having one of the most dominant seasons the sport's ever seen."

Capping one of the most impressive returns in tennis history, No. 2 Nadal toppled No. 1 Novak Djokovic 6-2, 3-6, 6-4, 6-1 on Monday to win his second U.S. Open and remain a perfect 22-0 on hardcourts in 2013.

At 27, Nadal stands in third place among men with 13 Grand Slam titles behind only the retired Pete Sampras (14) and Roger Federer (17).

Nadal, who watched the U.S. Open at home in his native Mallorca last year while resting his troubled knees, fell to his back on match point when Djokovic netted a forehand.

Soon after sharing a warm embrace at the net, Nadal was face down again on the cement in Arthur Ashe Stadium heaving sobs of joy and relief.

"For a few things this season (it's) probably the most emotional one in my career," eight-time French Open champion Nadal of his second New York crown in four years.

Djokovic and Nadal were meeting for an Open-era record 37th time and third in a U.S. Open final. Nadal won in 2010 and Djokovic took the title in 2011.

The 3-hour, 21-minute match - short by their typical standards - was a smorgasbord of awe-inspiring shotmaking and momentum shifts, personified by a 54-point rally won by the Djokovic in the second set.

The two most gifted defenders in a golden age of defense stretched, dove, skidded and used every fraction of the court with baseline blasts, drop shots, angles and lobs. It was tennis on Xbox.

"Nobody brings my game to my limit like Novak," Nadal said.

Down early, Djokovic came storming back, breaking Nadal twice in second set â?? the first time on the transcendent rally that brought the 23,000 fans in Arthur Ashe Stadium to their feet - and broke the Spaniard for a third consecutive time to open the third set.

Until the final, Nadal had dropped serve once in 82 attempts.

"When Novak plays that level, I am not sure if nobody can stop him," Nadal said.

But Djokovic played a sloppy game in to drop serve in the sixth game and let Nadal back in the door. The second-ranked Spaniard visibly pumped himself back up with fist pumps and shout of "Vamos!"

Nadal broke again to take the set and ran away in the fourth set as a flagging Djokovic lost his energy and conviction.

"He was too good," the six-time Grand Slam champion. "He definitely deserved to win this match today and this trophy."

Praised for his physical style of play, Nadal is as much brain as brawn. His comeback underscores his unique ability to adapt.

During his seventh-month layoff following Wimbledon, Nadal and his uncle/coach Toni Nadal went to work trying to figure out how to play more efficiently and challenge Djokovic, who entering the final trailed Nadal 21-15 in overall meetings but led 11-6 on hardcourts.

When Nadal won the 2009 Australian Open and 2010 U.S. Open, he did so with grip changes and power, especially on his serve. But realizing he had to shorten points to ease the wear-and-tear on his knees, he and his uncle decided instead to focus on court positioning.

Toni likened the tactical decision to move closer to the baseline and force the issue earlier in rallies to expanded aural abilities when sight is impaired.

"When you are blind, you hear so much more," he said Monday.

He moved closer to the baseline to take away time, pounced on short balls, approached the net more frequently and used his devastating forehand to take control of rallies early in the point.

He also improved his serve by changing direction, location and spins to elicit weak returns he could punish with his forehand.

He proved again that as the game's best problem solver.

"For me it's unbelievable that people talk about his body because he is so much better in his mind," said Uncle Toni, Nadal's only coach since childhood.

Mats Wilander, who comments for Eurosport, said he had never seen a player with Nadal's ability to problem solve, adapt and fight.

"I don't think anyone has played the game with the same kind positive energy and emotion as Rafa Nadal," said Wilander, comparing him to do-or-die competitors such as Lleyton Hewitt and Jimmy Connors. "He's a new breed, and we've never seen anything like it."

Top players have rarely come back from extended injury or mental breaks to reclaim their former form, John McEnroe and Wilander among them. During his mid-career tailspin, Andre Agassi plummeted outside the top 100 but managed to return to No. 1 and win multiple Grand Slam titles.

But no modern player has come back so quickly or efficiently as Nadal.

Since returning in February, Nadal has reached 12 of 13 finals, winning the French Open and U.S. Open while going a tour-best 60-3.

The one hiccup in his season was a first-round loss to 135th-ranked Steve Darcis at Wimbledon.

Time will tell if grass and not cement is Nadal's Achilles' heel.

The poorer footing and lower bounce puts more stress on the thighs and knees, and Nadal said as much after appearing to struggle physically in his All-England Club defeat.

Djokovic also faces questions. The 26-year-old had plenty of heart in Monday's final but often came up short in the clutch, something that has periodically plagued him in 2013 despite reaching three Grand Slam finals. Against Nadal, he was only 3-for-11 on break point opportunities.

"But it was obvious that in the important moments he played better tennis, and that's why he deserved to win," Djokovic said.

"I wish I won at least one title more, considering the fact I played two finals," added the Australian Open winner, who lost to Andy Murray in the Wimbledon final and to Nadal in five sets in the French Open semifinals after leading 4-2 in the final set. "All the matches I lost, even the French Open. I had that match."

Djokovic leaves New York clinging to his No. 1 ranking but aware the pecking order in men's tennis, and perhaps on his favored hardcourts, has shifted.

Nadal leaves 2013 with two Grand Slam titles and a nearly unassailable position to claim the year-end No. 1 ranking after missing all of last fall.

"What can I say?" said a resigned Djokovic. "He won so much this year. I'm still No. 1 of the world in the rankings, but year to year he's far, far ahead, and he has much more chances to end up as No. 1."

The hierarchy of the sport, however, has hardly changed.

The Big Four of Nadal, Djokovic, Andy Murray and Federer again won all four majors in 2013 and 34 of the past 35 Grand Slam titles.

Nadal and Djokovic, who have formed the most compelling rivalry in the men's game, have been the dominant players in recent years, with one or the other taking 12 of 15 since the 2010 French Open.

Gimelstob said if Nadal stays healthy he has a shot at catching Federer, who now has four more majors.

"He's viable on every surface," Gimelstob said.

1988 champ Wilander's only worry is that Nadal knows only one speed: full throttle.

"The important thing about Rafa," said Wilander, "is that he's going to play himself into the ground again and again."